1. Field of the Invention
The present invention relates to an acoustic wave filter device using surface acoustic waves or boundary acoustic waves and a duplexer. More specifically, the present invention relates to an acoustic wave filter device having a configuration in which interdigital transducers (IDTs) are withdrawal-weighted and a duplexer.
2. Description of the Related Art
Conventionally, various types of surface acoustic wave (SAW) filter devices have been used as bandpass filters of mobile communication apparatuses. The SAW filter devices include a plurality of longitudinally coupled resonator SAW filters connected in parallel.
For example, Japanese Unexamined Patent Application Publication No. 8-181566 (Patent Document 1) discloses a configuration in which first and second longitudinally coupled resonator SAW filter units are connected in parallel to an input terminal. Each of the first and second longitudinally coupled resonator SAW filter units is formed by cascading two longitudinally coupled resonator SAW filters having the same design in two stages.
In the acoustic wave filter device described in Patent Document 1, the crossing width of electrode fingers is half of that in a case where one longitudinally coupled resonator acoustic wave filter is used, and the two longitudinally coupled resonator acoustic wave filter units are connected in parallel to the input terminal. With this configuration, the resistance of the electrode fingers can be reduced to a quarter and the insertion loss of the acoustic wave filter device can be reduced.
On the other hand, Japanese Unexamined Patent Application Publication No. 2003-318700 (Patent Document 2) discloses a longitudinally coupled resonator SAW filter device having the electrode configuration shown in FIG. 12. As shown in FIG. 12, the longitudinally coupled resonator SAW filter device 501 described in Patent Document 2 includes first and second longitudinally coupled resonator SAW filters 503 and 504 connected in parallel to an input terminal 502. The longitudinally coupled resonator SAW filters 503 and 504 are connected to first and second balanced signal terminals 505 and 506, respectively.
Among the parallel-connected longitudinally coupled resonator SAW filters 503 and 504, the longitudinally coupled resonator SAW filter 503 is withdrawal-weighted. The other longitudinally coupled resonator SAW filter 504 is not withdrawal-weighted.
By withdrawal-weighting a plurality of IDTs of the longitudinally coupled resonator SAW filter 503, the degree of balance in a signal between the first and second balanced signal terminals 505 and 506 is improved.
In the longitudinally coupled resonator SAW filter device described in Patent Document 1, insertion loss can be reduced. However, it is difficult to obtain a sufficient attenuation in an attenuation band near a passband.
Also, in the longitudinally coupled resonator SAW filters described in Patent Document 2, an attenuation in an attenuation band near a passband is insufficient, although the degree of balance is improved by withdrawal weighting performed on the longitudinally coupled resonator SAW filter 503, as described above.